Date Published: March 23, 2017
Publisher: Public Library of Science
Author(s): Alexander Heil, Eduardo Lazo Gonzalez, Tim Hilgenfeld, Philipp Kickingereder, Martin Bendszus, Sabine Heiland, Ann-Kathrin Ozga, Andreas Sommer, Christopher J. Lux, Sebastian Zingler, Christoph Kleinschnitz.
The objective of this prospective study was to evaluate whether magnetic resonance imaging (MRI) is equivalent to lateral cephalometric radiographs (LCR, “gold standard”) in cephalometric analysis.
The applied MRI technique was optimized for short scanning time, high resolution, high contrast and geometric accuracy. Prior to orthodontic treatment, 20 patients (mean age ± SD, 13.95 years ± 5.34) received MRI and LCR. MRI datasets were postprocessed into lateral cephalograms. Cephalometric analysis was performed twice by two independent observers for both modalities with an interval of 4 weeks. Eight bilateral and 10 midsagittal landmarks were identified, and 24 widely used measurements (14 angles, 10 distances) were calculated. Statistical analysis was performed by using intraclass correlation coefficient (ICC), Bland-Altman analysis and two one-sided tests (TOST) within the predefined equivalence margin of ± 2°/mm.
Geometric accuracy of the MRI technique was confirmed by phantom measurements. Mean intraobserver ICC were 0.977/0.975 for MRI and 0.975/0.961 for LCR. Average interobserver ICC were 0.980 for MRI and 0.929 for LCR. Bland-Altman analysis showed high levels of agreement between the two modalities, bias range (mean ± SD) was -0.66 to 0.61 mm (0.06 ± 0.44) for distances and -1.33 to 1.14° (0.06 ± 0.71) for angles. Except for the interincisal angle (p = 0.17) all measurements were statistically equivalent (p < 0.05). This study demonstrates feasibility of orthodontic treatment planning without radiation exposure based on MRI. High-resolution isotropic MRI datasets can be transformed into lateral cephalograms allowing reliable measurements as applied in orthodontic routine with high concordance to the corresponding measurements on LCR.
Angular and linear measurements performed on lateral cephalometric radiographs (LCR) play a pivotal role in orthodontic routine diagnostics. Introduced in the 1930s  and further developed over many decades, lateral cephalometric analysis on LCR has remained the standard method in clinical routine until today. By assessing skeletal and dental relationships, it allows diagnosis and monitoring of various growth and development abnormalities . For example, lateral cephalometric analysis is important for the evaluation of severe skeletal malocclusions and for the planning of orthodontic appliances or orthognathic surgery [2, 3]. Radiation protection is of major importance in orthodontics, as the vast majority of patients are children or adolescents and as in most cases a series of radiographs is taken in the course of treatment. Because of the increased lifetime risk for stochastic radiation effects [4–6], it would be desirable to perform imaging in complete absence of ionizing radiation.
According to the ACR Phantom Test Guidance , all seven measurements performed with the MRI sequence used in the study (Table 1) were congruent with the known values of the ACR Phantom.
In particular in children and adolescents, avoidance of radiation exposure is crucial. In this study, we aimed to show equivalence of MRI to radiographs in lateral cephalometry as a basis for orthodontic treatment planning. To our knowledge, MRI based standardized lateral cephalometric analysis including midsagittal as well as bilateral landmarks has not been evaluated before. An isotropic T1-weighted sequence with excellent contrast, high spatial resolution and short scanning time formed the basis for our new approach. Images yielded from this MRI technique allowed a clear depiction of the dental and skeletal cephalometric landmarks. The subsequent postprocessing algorithm enabled the transformation of the isotropic MRI datasets into lateral cephalograms covering the midsagittal and bilateral landmarks necessary for diagnostics and treatment planning in orthodontics. Based on these generated lateral MRI cephalograms it was possible to perform a detailed cephalometric analysis with a broad spectrum of measurements as used in orthodontic routine. Linear and angular cephalometric measurements taken on lateral MRI cephalograms turned out to be highly reliable as interobserver and intraobserver agreement was excellent. As a principal finding, we found high levels of agreement between the measurements on lateral MRI cephalograms and the corresponding measurements on LCR in a clinical environment by examining young patients with various orthodontic abnormalities. Statistical equivalence between the two modalities was shown for 23 out of 24 measured distances and angles within a strict predefined equivalence margin of ± 2 mm / ± 2°. The only measurement without statistical equivalence was the interincisal angle, which also showed a slightly higher bias level in Bland-Altman analysis in comparison to the other cephalometric measurements. This, however, was not an unexpected finding, as the interincisal angle is prone to measurement errors when performed on LCR [22, 23]. Nonetheless, the mean difference of -1.33° in Bland-Altman analysis still indicated a low and clinically tolerable bias for the interincisal angle. Considering the overall high concordance with LCR (“gold standard”) and the absence of radiation exposure, lateral cephalometric analysis for the assessment and monitoring of orthodontic conditions could be performed by MRI in the future to keep radiation dose in young patients as low as possible.
In conclusion, this study shows that full lateral cephalometric analysis as applied in orthodontics is feasible based on postprocessed MRI datasets. There was a high concordance with equivalent measurements taken on LCR, which is the standard method in clinical routine. Our MRI based approach for the first time enables the assessment of orthodontic conditions by using clinically standardized analysis methods in absence of radiation exposure to the mostly young patients. The short and well-tolerated examination protocol applied in our feasibility study could be integrated into clinical routine. Further studies with large patient populations using different MRI systems should be conducted to support our findings and to evaluate whether MRI and LCR are equivalent in lateral cephalometric analysis under the most diverse clinical and technical conditions. Moreover, our MRI technique has the potential to overcome the limitations of projection radiography in the future.